A vacuum countergravity casting process using a gas permeable, self-supporting mold sealingly received in a vacuum housing is described in such patents as the Chandley et al U.S. Pat. Nos. 4,340,108 issued July 20, 1982 and 4,606,396 issued Aug. 19, 1986. That countergravity casting process involves providing a mold having a porous, gas permeable upper mold member (cope) and a lower mold member (drag) sealingly engaged together at a horizontal parting plane, sealing the mouth of a vacuum housing to a surface of the mold such that a vacuum chamber formed in the housing confronts the gas permeable upper mold member, submerging the bottom side of the lower mold member in an underlying molten metal pool and evacuating the vacuum chamber to draw molten metal through one or more in gate passages in the lower mold member and into one or more mold cavities formed between the upper and lower mold members.
The mold and the vacuum housing typically are sealed together using a gasket seal compressed between the bottom lip of the vacuum housing and an upwardly facing sealing surface or flange formed on the mold, either on the lower or upper mold member. Various mechanical clamping mechanisms have been provided for clamping the vacuum housing and the mold together to compress the seal therebetween; e.g., as shown in U.S. Pat. Nos. 4,340,108; 4,616,691 and 4,658,880.
The need for such mold-to-vacuum housing sealing systems complicates the casting apparatus as well as the casting mold. In this latter regard, the mold must include the sealing surface/flange needed to cooperate with the gasket seal and oftentimes attachment features, such as threaded lugs, needed to cooperate with the mechanical clamping mechanism. Moreover, the need for such mechanical sealing systems limits to some extent the variety of mold designs which can be used with the system.
In the countergravity casting process described in the aforementioned patents, the lower and upper mold members typically are engaged at a horizontal parting plane therebetween. Engagement of the lower and upper mold members at the parting plane is effected in such a manner as to substantially prevent or minimize leakage of molten metal from the mold cavity at the parting plane during casting since molten metal leakage can result in the production of unacceptable castings and damage to the vacuum housing and associated vacuum components of the casting apparatus. To this end, the lower and upper mold members are often adhered (e.g., glued) together at the horizontal mold parting plane. The gluing process for sealingly engaging the upper and lower mold members together is expensive and time consuming and elimination thereof would improve the efficiency and economies of the vacuum countergravity casting process.
In practicing the aforementioned vacuum countergravity process, the mold is subjected to flexural and other stresses when the vacuum chamber confronting the upper mold member is evacuated and the molten metal is drawn upwardly into the mold cavity. The thickness and thus the strength of the walls of the casting mold must be sufficient to withstand these and other stresses imposed on the mold during casting to prevent cracking or total fracture of the mold and resultant molten metal leakage from the mold cavity into the vacuum chamber. A reduction in both the thickness of the mold walls and the outside structural features needed for sealing to the mouth of the vacuum chamber would reduce the amount of expensive resin-bonded sand employed in the mold and also improve the economies of the casting process. Moreover, without such excess mold material and structural features, more of the volume of the vacuum chamber would be available to accommodate more molds and hence increase the number of castings possible per casting cycle for a given size vacuum chamber.
It is an object of this invention to provide an improved, economical countergravity casting apparatus and process using a gas permeable, self-supporting mold (e.g., a resin-bonded sand mold) which does not require a mechanical mold-to-vacuum housing sealing/clamping system.
It is another object of this invention to provide an improved, economical countergravity casting apparatus and process using a gas permeable, self-supporting mold wherein the amount of costly resin-bonded mold particulate (e.g., resin-bonded sand) required for the mold is substantially less than heretofore required.
It is another object of this invention to provide an improved, economical countergravity casting apparatus and process using gas permeable, self-supporting molds wherein more molds/mold cavities are possible per given size vacuum chamber than heretofore possible, thereby resulting in substantially increased productivity and economies.
It is a further object of the invention to provide such an improved, economical countergravity casting apparatus and process wherein a gas-permeable, self-supporting mold is disposed in an open bottom container with a particulate bed compacted about the mold and wherein the mold and the particulate bed are subjected to a negative differential pressure in the container in such a manner as to hold the particulate bed about the mold and preferably also hold the mold in an inverted casting position in the container before, during and after filling with the molten metal.
It is a further object of the invention to provide an improved, economical countergravity casting apparatus and process using a gas permeable, self-supporting mold which includes a plurality of mold members stacked side-by-side and configured at parting planes therebetween to provide a significantly increased number of mold cavities available for casting per mold.
It is still another object of the invention to provide an improved, economical countergravity casting apparatus and process of the preceding paragraph wherein the mold members are held in stacked side-by-side relation in such a manner as to eliminate the need to glue the mold members together at the parting planes therebetween.